77 xs 360 carb not coming back down to idle

[monolith]

hello all. i have a 77 xs360 with some carb troubles. i had some trouble with the bike revving really high and not coming back down to idle. so i just soaked the carbs over night in pine sol solution. it worked amazingly! they are pristine! but still having problems, although its better, with the bike not coming back down to idle. my thinking is one of the butterflies is staying open just enough to let more air in than the other. the throttle cable returns nicely, i lubed the cable really well.

if i rev the bike up to 3000 rpm it stays there and i have to kill the bike. i thought it may be a sticking slider but they both seem to return just fine. the only other thing i that will bring it back down is if i ever so slightly turn the linkage that extends from the outer portion of the carb. its really more of a bump...it is the portion that contains the spring. the linkage is very clean and i thought soaking everything would do the trick but unfortunately it did not. if i loosen the nut on the end of the linkage the bike comes down but very slow.

so all in all, this is why my thinking is leading towards one of the butterflies being slightly off. there has to be excess air getting in there somewhere? can anyone confirm im not crazy? thanks!

[HoughMade]

The first thing i would look for is an intake leak. Even if there is no air leak, the engine can hang at a high rpm when revved if it is too lean (or too rich), but with a lean condition, you would probably getting a popping at idle after the engine is warm. Third, synchronize the carbs if that has not been done recently. I really doubt that a butterfly is staying open- unless it's out of balance. You should be able to tell that by looking at both throttle shafts. after you let off the throttle.

[monolith]

yeah i had a feeling it may be an intake leak but its weird that specifically the throttle linkage changes it. i will go ahead and try synching the carbs again and see if i can get it to stay down.

[HoughMade]

Sometimes there can be air leaks around the butterfly shafts themselves when the holes in which they are mounted wear slightly out of round.

[monolith]

sure that makes sense. how does one remedy that? maybe i will spray around those openings and see if that has an effect on how the bike runs. that way i will know...question is, am i looking at a new seal on the opening?

[HoughMade]

Yep, I would test it just the way you say. The seals are $5 for a set at Mike's XS if that's it.

[monolith]

Yep, I would test it just the way you say. The seals are $5 for a set at Mike's XS if that's it.

believe it or not slightly adjusting the pilot screws really helped, in fact, it comes back down at a normal speed and doesnt want to stay up unless i hold the throttle. i guess i didnt assume before this would work, but it did. ill keep an eye on it and make sure it remains there but from here it seems pretty damn well solid.

[HoughMade]

That's good to hear. I had a similar issue (as you may have surmised) but mine was popping at idle. I balanced the carbs and richened the idle mixture up and the problem went away.

[monolith]

That's good to hear. I had a similar issue (as you may have surmised) but mine was popping at idle. I balanced the carbs and richened the idle mixture up and the problem went away.

yeah man, thanks for your help. by the way if you ever have to clean your carbs, i highly recommend the pine sol treatment, its quite amazing. you may have your own method but mine were filthy on the outside and they now look brand new. i throw some pics up of the bike when the seat is done.

[HoughMade]

I look forward to seeing the pics.

[dice1649]

hello all. i have a 77 xs360 with some carb troubles. i had some trouble with the bike revving really high and not coming back down to idle. so i just soaked the carbs over night in pine sol solution. it worked amazingly! they are pristine! but still having problems, although its better, with the bike not coming back down to idle. my thinking is one of the butterflies is staying open just enough to let more air in than the other. the throttle cable returns nicely, i lubed the cable really well.

if i rev the bike up to 3000 rpm it stays there and i have to kill the bike. i thought it may be a sticking slider but they both seem to return just fine. the only other thing i that will bring it back down is if i ever so slightly turn the linkage that extends from the outer portion of the carb. its really more of a bump...it is the portion that contains the spring. the linkage is very clean and i thought soaking everything would do the trick but unfortunately it did not. if i loosen the nut on the end of the linkage the bike comes down but very slow.

so all in all, this is why my thinking is leading towards one of the butterflies being slightly off. there has to be excess air getting in there somewhere? can anyone confirm im not crazy? thanks!

Hey,

i have this same problem with my 1976 Yamaha XS360. once my bike warms up, the revs will shoot up to about 4000rpms and just stay there until i kill it.

i used brake cleaner spray to locate an air leak in the "linkage" that you mentioned. spraying break cleaner on the spring made the bike drop from 4000rpm back down to a smooth 1200. but then in a few seconds, its back up to 4000. also, like you said, if i turn the linkage counterclockwise, the revs come back down to 1200.

when i found the air leak with the break cleaner, i thought all i needed to do was buy new butterfly throttle shaft seals from Mikes XS, install them, and I'd be good to go. but if my problem is being caused by bad o-rings, why would turning the linkage counterclockwise drop the rpms back down to 1200?

i took the carbs off the bike tonight and i can see a lttle more light coming in from the left butterfly than the right one. and then if i turn the linkage counterclockwise, i can hardly see any light coming through at all.

can i assume that my problem is that the butterflies are just not closing tight enough? maybe they're letting enough air sneak through to push the revs up to 4000?

what do you guys think?